1. Field of the Invention
This invention relates to compositions useful as improved pressure sensitive hot melt adhesives and for other applications. As novel hot melt adhesives these compositions display low melt viscosity, simplifying application, as well as improved adhesive properties, especially shear resistance. These compositions are comprised of a polymer containing copolymerized organic acid, a miscible metal salt, and an o-methoxy-aryl acid.
2. Brief Description of the Prior Art
Hot melt pressure sensitive adhesives are tacky, soft materials at room temperature. In order to apply these materials to substrates such as paper, plastic, or cloth, these adhesives are heated to about 250.degree.-350.degree. F. in order to lower their viscosity so that the substrate may be easily coated. The adhesive coated substrates may then be used as, or used in the manufacture of, a variety of useful articles such as tapes, labels, decals, et al. A significant limitation on uses for which hot melt pressure sensitive adhesives may be employed is the relatively low shear resistance of these materials. Although these materials have been traditionally described as solids at room temperature, this description is in a sense erroneous. These adhesives are based on primarily thermoplastic polymers. Thus, although they can exhibit very high viscosity at room temperature, giving the appearance of soft materials, they will flow under an applied stress. Thus they are more accurately described as high viscosity polymeric liquids at room temperature. This nature is reflected in the effect of changing temperature on the viscosity of these materials. As temperature increases there is a gradual reduction of viscosity and the materials ultimately attain a viscosity low enough to make coating operations practical. After application, a temperature decline is reflected in the corresponding increase in the viscosity of the material. An ideal hot melt pressure sensitive adhesive would exhibit the mechanical properties of a true solid at room temperature while simultaneously retaining the ability to flow easily at the elevated application temperature. Although thermosetting materials as a class have proven impractical for this application, ideally, the pressure sensitive hot melt adhesive would behave as a crosslinked thermoset resin at room temperature: the application of shear stress would result merely in deformation and not in flow of the adhesive. Thus the viscosity of the ideal material would change from essentially infinite at room temperature to very low at the application temperature. Thus, in a certain sense the magnitude of this viscosity difference is a measure of quality of the hot melt adhesive.
One approach that has been used in the past to maximize this viscosity difference has been the incorporation of small amounts of functional monomers in the adhesive polymer coupled with the use of additional components to help form thermally reversible crosslinks between the polymer chains. For example, U.S. Pat. No. 3,925,282, granted Dec. 9, 1975, to Davis et al., disclosed acrylic pressure sensitive hot melt adhesive compositions which contain a tertiary amine-containing monomer in the acrylic copolymer. In addition to the copolymer, these compositions contain an organo metallic salt which is capable of forming reversible coordinate crosslinks. At room temperature the presence of these crosslinks causes the hot melt adhesives to exhibit improved cohesive strength properties and increased shear resistance corresponding to higher viscosity relative to the uncrosslinked material. However, at high temperature the crosslinks are broken and the material flows easily. Another type of reversible crosslinking known to the art of pressure sensitive adhesives is effected when these compositions include a divalent metal ion such as zinc or zirconium and carboxylic acid residues attached to the polymer chain, as disclosed, for example, in U.S. Pat. No. 4,145,467, granted Mar. 20, 1979, to F. T. Sanderson et al.
Another approach has been to include photo initiators in the hot melt composition which can be photopolymerized in situ after application of the adhesive to the substrate. This approach is exemplified by U.S. Pat. No. 4,052,527, granted Oct. 4, 1977, to S. D. Pastor et al., in which photoinitiators are interpolymerized with acrylic copolymers to produce ethylenically unsaturated prepolymers. These are applied in hot melt form to the substrate and are subsequently cured by exposure to ultraviolet radiation. This approach suffers from the drawbacks of requiring: (1) expensive photoinitiators, (2) ultraviolet sensitivity prior to cure, and (3) an additional cure step requiring additional process equipment.
A review of the state of the hot melt adhesive art is presented by the materials collected by D. L. Bateman in Hot Melt Adhesives (Noyes Data Corp., Third Edition, 1978).